Claim and Claim Adjustment Expense Reserves (Specialty - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development) (Details) - Specialty - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	$ 10	$ 15
Other Professional Liability and Management Liability	3	(12)
Surety	(30)	(25)
Warranty	0	0
Other	6	2
Total pretax (favorable) unfavorable development	$ (11)	$ (20)